In cellular communications networks, Multiple-Input and Multiple-Output (MIMO) systems facilitate a radio link using multiple transmit and receive antennas. These may be used to increase the reliability of a transmission by taking advantage of multichannel transmissions or by providing spatial diversity. MIMO systems may also be used to increase the throughput of a radio link by transmitting more information at the same time. To assist downlink scheduling in a MIMO system, a wireless device will report at least one of a Rank Indicator (RI), downlink Channel Quality Indicator (CQI) report, Hybrid Automatic Repeat Request (HARQ) Acknowledgement (ACK) and/or Negative Acknowledgement (NACK) feedback. A radio access node, in determining the number of layers to use to perform a downlink transmission to the wireless device, needs to consider the reported RI, CQI, and HARQ ACK/NACK feedback.
There may be times when the RI reported by a wireless device is not accurate. Some factors affecting the accuracy of the RI include an abrupt change in channel condition. The rank after the change may be different from a previously reported rank. Often RI and CQI reporting is scheduled periodically, and a period is chosen to make a compromise between the need for more frequent reports in time-varying wireless channels and the uplink resources to be consumed by the reporting. That means more frequent reporting cannot be scheduled to cover this abrupt channel condition change.
Another reason the RI reported by a wireless device may not be accurate is that the wireless device may be using device specific rank determination algorithms. For instance, the 3rd Generation Partnership Program (3GPP) Technical Specification (TS) 36.101 defines minimum performance requirements regarding rank reporting, but those are only minimum requirements. For different vendors, device specific rank determination algorithms may not use the same criteria, and they may not necessarily maximize the throughput.
The RI reported by a wireless device also may not be accurate because of inter-cell interference. Wireless devices commonly use Cell-specific Reference Signals (CRS) across several subframes to determine a rank and an associated CQI. Due to the fact that inter-cell interference may not affect the Physical Downlink Shared Channel (PDSCH) region to the same degree that the inter-cell interference affects CRS, the rank estimated based on CRS could be aggressive or conservative, not reflecting the actual PDSCH decoding capability.
Antenna correlation can also lead to the RI reported by a wireless device being inaccurate. Antenna correlation will cause a MIMO channel matrix to have a large eigenvalue spread, meaning layers will have different Signal-to-Noise Ratios (SNRs). Wireless devices may use this eigenvalue spread to determine a rank. If the eigenvalue spread is larger than a threshold, a step-down rank may be determined.
An inaccurate RI reported by a wireless device may cause the radio access node to transmit more data than the channel conditions will support. This can cause a reduction in the downlink throughput from the radio access node to the wireless device. As such, there is a need for systems and methods of operating a radio access node in a cellular communications network to compensate for the inaccuracy of the RI reported by a wireless device.